Comparison of Smith/Nomarski DIC and PlasDIC

A: Smith/Nomarski Differential Interference Contrast. The action of Nomarski Prism 1 (Nom1) and the Köhler condenser creates two "difference image plane waves" from a single polarized wave (from Pol). These two waves create two phase-distorted images ("difference images") after passing through a transparent object with a refractive index different from the surrounding medium. The distance between wave pairs is called shear (delta) and the phase offset between wave pairs is called bias (lower case delta). A second, inverted Nomarski prism (Nom2) combines the two sheared wavefronts and the second polarizer (analyzer) folds the vibrational planes into a single plane. It's at this point, the Analyzer, that wave interference occurs. Both prisms are set at 45° to the orientation of the two polarizers. This allows both the o- and e-wavefronts to interact at the Analyzer.

B: PlasDIC works under the same general concept as DIC (above) except that the Polarizer and Nomarski-modified Wollaston prism are placed after the (birefringent) specimen has induced a phase distortion to the impinging wavefront. A Nomarski-modified Wollaston prism splits the wavefront into two spatially separated wavefronts, which then pass through a polarizer. Since the Prism is oriented at 45° to the plane of the Analyzer, both wavefronts are modified to vibrate in the same plane. Interference occurs only when the two wavefronts become coherent at the Intermediate Image Plane (not shown). Unfortunately PlasDIC requires a slit illuminator (oriented parallel to the prism) and low magnification/NA objectives, thus magnification is limited to <40x objective magnification. However, it is an excellent optical technique for imaging cells growing on plastic petri plates.